Motor vehicles often use monopole antenna structures, such as the telescopically extended whip antenna, to provide an acceptable level of reception of radio signals in a moving automobile. However, the extension or protrusion of such structures from the vehicle body, in order to effectively receive the signal, exposes the antenna to abuse or inadvertent contact with debris during travel, induces wind noise and must be modified with complex and expensive retraction gear in order to protect the antenna when not in use. Moreover, retractable antennas mechanisms require substantial storage space and raise substantial packaging problems.
Concealed or conformal antenna structures such as those embedded in window lites of a motor vehicle, have been installed in order to overcome the problems discussed above. However, such antennas have had difficulty in meeting the performance standards of the conventional monopole aerial antenna. Substantial efforts have been undertaken and continue to be invested to improve radio reception using such antennas. Because numerous parts of the vehicle body, for example conductive panels made of sheet metal, and numerous systems including electrical circuits throughout the vehicle can interfere with the reception of electromagnetic radio waves to be received by the antennas. Moreover, particular alignments of concealed conductors often provide a limited range or direction of reception, and thus find only limited usefulness in motor vehicle applications.
In addition, while it is known that the diameter of the antenna can be increased in order to improve the gain characteristics of the antenna across a larger bandwidth, the increased breadth of the conductor covers a larger portion of the window lite in which it may be mounted. Accordingly, antennas that are structured to improve performance according to conventional principles of antenna structure theory are quite unsuitable for concealing or obstructing the window lites in motor vehicles.
It has also been known to use dipole antennas. However, since dipoles are resonant type structures, the bandwidth of signals that they can receive is relatively low. Nevertheless, while it is known to make the conductor of the dipole antenna thicker to increase bandwidth and lower VSWR (voltage standing wave ratio), thickening of the antenna adversely affects the ability to conceal the antenna. As a result, limited success has been attained with the conventional changes of length and thickness of antenna conductors. In addition, changes in thickness are not an attractive option on window lites. Accordingly, the gain of previously known concealed window lite FM antennas has been limited, particularly at the ends of the bandwidth of the FM broadcast spectrum.